Charging characteristics for nickel cadmium (NICD) batteries are shown in curve b of FIG. 5. The battery voltage increases slowly for 90% of the battery capacity during charging, but then starts to climb steeply after that, from point P1 onwards. At point P3 the voltage levels off and starts to decline from point P3 to point P4. It is known to attempt to determine the end of charge point by detecting this voltage drop (-dV) at the end of charging.
Due to increasing usage of ultra-fast or fast charging rates (one hour or less than one hour charging time) in present day high end chargers, switch mode power supply technology is employed for the design of chargers due to reasons of size and efficiency. However, these designs have high switching noise.
These switching noises can take the form as shown in FIG. 8a. They can be spikes or glitches S on an otherwise calm and slowly changing battery voltage level V.sub.batt. False detection by a conventional charger monitor using a -dv method can occur in cases like that depicted in FIGS. 8b and 8c. In FIG. 8b, a noise spike S reaches a voltage level v1. If one voltage measurement is made at v1 and another at v2, a -dV detection will be indicated if the measurement at v2 is lower than that at v1 by more than the preset threshold, e.g. 50 mV. The same false detection will occur if, as in FIG. 8c, the measurement at noise spike v4 is lower than that at v3 by more than the same threshold value (FIG. 8c).
Due to the fact that amplitudes of switching noises S are generally in the range of hundreds of mV, whereas the detection threshold of -dV detection is only tens of mV, to avoid false detection due to these noise signals, extensive filtering has to be used. Very often, too much filtering causes a slow response and the result is that the battery is overcharged before the -dv point is detected and charging terminated. The avoidance of noise by filtering is also very dependent on the particular design of charger. Optimum filtering for one charger may be insufficient for another. It also increases the cost of implementation.
Another known method to avoid false detection in -dV detection techniques is to interrupt charging and to measure the battery voltage before resuming charging. This is done because during the interval when charging is stopped, switching noises are minimal and it is thus an optimum time to take a measurement of battery voltage. The drawback of this approach is that charging times are considerably longer due to the interruptions and also delays are caused due to the waiting times during voltage measurements. The waiting times are necessary for the battery voltage to settle down just after charge interruptions to ensure accurate readings.
The aim of the present invention is to enable the -dV point to be detected accurately at the end of charge when charging on an NICD battery in a noisy environment.